This invention relates to devices for stepping up or stepping down the speed of rotation of a micromotor, particularly micromotors for dental use, whether electric or pneumatic. More particularly, the present invention relates to a device for providing two different speed ratios at the motor's output, and to handpieces and micromotors fitted with such a device. Although the disclosed device is primarily intended for motors it is also applicable to other devices calling for similar speed control.
Dental practice often requires very different ranges of rotational speed for the tool (e.g., a drill) depending on the operation to be performed. For example, for drilling operations, the speed of rotation will correspond to the maximum speed of the motor, whereas for prophylactic operations, the necessary speed will correspond to the minimum speed of the motor.
It is of course possible to cause the speed of an electric motor to vary by adjusting the supply device, and to vary the speed of a pneumatic motor by a device such as that described in French Patent 2,454,513. The disadvantage is that in all cases, and in particular for pneumatic motors, the torque characteristic falls away considerably at low speed. This disadvantage can be palliated by placing a straight or reverse-angled handpiece on the motor. This ensures a speed reduction so that the motor can turn at an acceptable speed. However, the practitioner is then constrained to have two types of handpieces.